1. Field of the Invention
The subject invention relates to television receivers, and more particularly, to preventing the beam current in television receivers from exceeding a predefined maximum amount.
2. Description of the Related Art
FIG. 1 shows the high voltage circuitry in a known projection television receiver. The high voltage output/driver circuit 10, under the control of a high voltage drive pulse, applies a B+ voltage of 130 V. to a primary winding L1 of a high voltage transformer 20, the primary winding L1 being connected to ground via a capacitor C1. The high voltage transformer 20 includes a plurality of secondary windings L2-L5 serially connected by diodes D1-D3. A diode D4 connects the free end of secondary winding L5 to the input of a high voltage splitter 30 which applies anode voltages to the red, green and blue projection tubes (not shown). Diode D2, in addition to being connected to the secondary winding L4, is also connected to a main focus output 22 of the high voltage transformer 20. A power supply voltage Vb is connected to the free end of secondary winding L5 via a series resistor R1 and across a capacitor C2 connected to ground, and supplies the beam current Ib. This beam current Ib is also supplied to an automatic beam current limiting (ABL) circuit (not shown)
Virtually every television receiver has an automatic beam current limiting, ABL, circuit. Its function is to limit the maximum beam current that is necessary to produce a bright picture on the screen when the brightness and the picture (contrast) controls are at maximum. In general, the object of an ABL circuit is to keep the maximum beam current below, for example, 1.8 ma. U.S. Pat. No. 4,017,681 shows a typical beam current limiting circuit.
When the brightness or contrast increases, more beam current, Ib, is drawn from the power supply, Vb, through the resistor R1 into the secondary winding L5 of the high voltage transformer 20, the high voltage splitter 30 and the anodes of projection tubes. Any amount of beam current in excess of, for example, 1.8 ma should be kept to a minimum so that the high voltage transformer 20 and the projection tubes will operate reliably. Otherwise, the high voltage transformer 20 and/or the projection tubes may fail leading to the costs for replacing these components which are expensive. In addition, too much beam current may cause the projection tubes to emit too much X-ray radiation which is harmful to health. To this end, the ABL circuit detects the power supply voltage Vb and correspondingly limits the luminance and chrominance drive circuits which, in turn, limits the brightness and contrast of the television receiver.
When the ABL circuit fails due to, for example, a failed component therein, or when the picture tube is zero biased, or both, a large amount of beam current over 1.8 ma will result. The high voltage output circuit C should then be disabled. As such, there will be no beam current. An over-current protection circuit is used to disable the high voltage output circuit when the beam current exceeds the maximum beam current by a certain percentage.
FIG. 1 shows an over-current protection circuit which uses a switching circuit 40 (e.g., IC SC78130P1 by Motorola) to disables the high voltage output circuit. In particular, the high voltage transformer 20 includes an auxiliary winding L6 which applies a flyback pulse, through a series arrangement of a diode D5 and a resistor R2, to input 3 of the switching circuit 40. Input 3 is connected to ground via a resistor R3 and a capacitor C3, and is also connected to an input 1 via a resistor R4. Input 4 is connected to ground and inputs 2 and 8 are connected to ground via capacitors C4 and C5, respectively. A supply voltage of +15 V. is applied to input 8 via a resistor R5. In addition, an on/off signal is applied to input 7 via a resistor R6. Finally, an output 8 of the switching circuit 40 is connected to a control input of the high voltage output/drive circuit 10.
The over-current protection circuit of FIG. 1 relies on the change of the voltage amplitude of the flyback pulse on the auxiliary winding L6 of the high voltage transformer 20. When the beam current increases, the amplitude of the flyback pulse decreases. When there is excessive beam current over 1.8 ma, the voltage amplitude of the flyback pulse decreases to such a low amplitude that, after it is rectified by diode D5 and the capacitor C3, the d.c. voltage at input 1 is less than the reference voltage at input 2. This actives the switching circuit 40 and causes the high voltage output/drive circuit 10 to shut down and to be latched.
Applicant notes, however, that this system does not monitor the beam current directly because the change of voltage amplitude of the flyback pulse at the auxiliary winding L6 of the high voltage transformer 20 does not directly track the exact change of the beam current. Furthermore, the amplitude of the flyback pulse will change due to the changing of the flyback time or due to the tolerances between transformers. Therefore, the exact amount of beam current over 1.8 ma cannot be tightly controlled and predicted precisely.